/* 进场动画开始 */
.alert-enter,.alert-appear {
  opacity: 0;
}
/* 进场动画进行中 */
.alert-enter-active {
  opacity: 1;
  transition: opacity 300ms ease-in;
}
/* 出场动画开始 */
.alert-exit {
  opacity: 1;
}
/* 出场动画进行中 */
.alert-exit-active {
  opacity: 0;
  transition: opacity 300ms ease-in;
}